Ponte da Arrábida

The Arrábida Bridge is a single concrete arch spanning 270 metres across the Douro, and when it was completed in 1963 it was the longest concrete arch bridge in the world. Designed by Edgar Cardoso — Portugal's greatest bridge engineer — it's a masterpiece of mid-century structural design that makes the Dom Luís I Bridge look fussy by comparison.

The bridge carries six lanes of traffic and was built to connect Porto to the beaches south of the river, but its recent fame comes from the Porto Bridge Climb — a guided walk over the top of the arch, 65 metres above the water, that's become one of the city's most popular experiences. You clip into a safety harness and walk the curved surface of the arch from one bank to the other, with views upriver toward the city centre and downriver toward the Atlantic. It's vertigo-inducing and exhilarating in equal measure.

Even from below, the bridge is worth seeing. The curve of the arch is mathematically elegant — a parabola that distributes the load with an efficiency that still impresses structural engineers. The best view is from the Foz promenade looking upriver, where the bridge frames the Douro valley like a gateway between the city and the ocean.